Condition-responsive system



Jan. 31, 1961 WOODWARD 2,970,201

CONDITION-RESPONSIVE SYSTEM Filed NOV. 24, 19 58 INVENTOR. 6473/ )i Ma-W CONDITION-RESPONSIVE SYSTEM Gary F. Woodward, Ann Arbor, Mich,assignor to King- Seeley Corporation, Ann Arbor, Mich, a corporation ofMichigan Filed Nov. 24,1958, Ser. No. 776,089

25 Claims. (Cl. 219-r2ii) This invention relates to temperaturecontrolling systems and more particularly to apparatus for controllingthe operation of a heater in accordance with the sensed temperature of abody.

This invention is an improvement upon the systems disclosed in theUnited States patent application of George B. Whinery, Serial No.604,867, filed August 20, 1956, in the United States patent applicationof Harry I. Baker, Serial No. 773,263 filed on November 12, 1958, and inthe United States patent application of Tore B. Hanssen, Serial No.773,286, also filed on November 12, 1958, and the disclosures of thosepatent applications are incorporated herein by reference.

The principles of the present invention are embodied in a system forcontrolling the operation of a heater disposed in heat transferrelationship with a body of air such as in the air in an oven or abuilding, in heat transfer relationship with a container body such as apan disposed in heat transfer relation with the surface heating elementon a stove, or in heat transfer relationship with a body to be cooked,melted or cured such as meat in an oven, food in a pan on the stove, orchemical materials being processed. Certain of the principles of theinvention are applicable to temperature or other types of gauging andthe term control as employed herein is intended to be generic toindication or gauging.

In general, in the disclosed arrangements, the energization of theheater is controlled in accordance with the temperature of the body asdetermined by a sensing element disposed in heat transfer relation witha body, normally the body being heated, with that sensing elementpreferably taking the form of a resistor having a high temperaturecoefficient of resistance.

In the preferred arrangement, the variable-resistance senser is utilizedto vary the effective output voltage of a slef-interrupting or pulsingthermal relay in accordance with the sensed temperature of the body. Thepulsing relay is normally arranged to deliver output energy as a seriesor succession of pulsations the effective voltage of which issubstantially independent of variations in the voltage of the source ofelectrical energy for the control system. The output energy from thepulsing relay is applied to a control or responder relay which, in theillustrated embodiment, responds only to the effective value of theapplied voltage rather than to the individual pulsations. The responderrelay in turn controls an output relay which controls the energizationof the heater.

In accordance with certain of the principles of the present invention,means in the form of an auxiliary heat ing element disposed in heattransfer relationship with the pulsing relay are provided to establish apositive feedback or amplifying conditon as a result of which thevariation in the effective output voltage of the pulsing thermal relayin response to unit change in the resistance of the senser issubstantially magnified.

The principles, objects and features of the invention will best beperceived from the following detailed descripf; o as tion of embodimentsof the invention when read with reference to the accompanying drawingsin which:

Figure 1 is a schematic representation of a temperature controllingsystem embodying principles of the present invention; and

Fig. 2 is a schematic representation of a modified temperaturecontrolling system also embodying principles of the present invention.

The system disclosed in Fig.1 of the drawings comprises a pulser 10controlled by a senser 12 and controlling a responder 14 which controlsan output relay 16 to control the energization of a load 18 from asource of energy 20, the control system also being energized from thesource 21 through a transformer 22.

The senser 12 is preferably a resistor (or a plurality of resistors)having a high temperature coefiicient of resistance and disposed in heattransfer relationship with a body. Senser 12 may be constructed of wireand have a high positive temperature coefiicient of resistance or maybe, for example, a ceramic thermistor element having a high negativetemperature coefficient of resistance, it being assumed in theillustrated arrangement that a senser having a high positive temperaturecofiicient of resistance is employed.

Intimate heat transfer relationship between the senser 12 and the body,the temperature of which is being sensed, may be established in anyappropriate fashion. For example, the senser may be disposed within apointed probe for insertion in meat in an oven or in food being cookedin a pan; it may be disposed in a room or in an oven for sensing thetemperature of the air; or it may be disposed at the center of a surfaceheating element in a stove to sense the temperature of the bottom of apan disposed upon the surf-ace element. An example of a senser unit forthe latter purpose and suitable for utilization as an element of thesubject system is disclosed in the United States patent application ofE. E. Sivacek, Serial No. 604,917, filed August 20, 1956, and thedisclosure of that application is incorporated herein by reference.

The load 18 is herein assumed to be a heating element of the electricalresistance type. Heater 18 will normally, but not necessarily, bedisposed in heat transfer relationship with the same body, thetemperature of which is being sensed by senser 12.

The source 20 which supplies the electrical energy for the controlsystem may be, for example, a line source of alternating voltage such asis found in the home and is represented as a three-line source, with anominal volt alternating potential existing between lines 20a and Zilb(the neutral) and with a nominal 230-volt alternating potential existingbetween lines 20a and 200. As is well known, the line voltage is subjectto substantial varia tion and accordingly, one of the functions ofpulsing device 10 is to prevent such variations of line voltage fromadversely affecting the consistency of operation of the equipment.

Upon the closure of switch 21, the alternating voltage between lines 20aand 2% is applied across the primary winding of step-down transformer 22so that a reducedmagnitude voltage appears across the secondary windingof that transformer and hence between conductors 26- and 28. Thisvoltage is applied through the senser 12 to the electro-thermal voltageregulating device 10, one function of which is to receive the noticeablyvariable voltage between conductors 26 and Z8 and to deliver pulsatingenergy to the device 14 having an effective value which is substantiallyindependent of the variations in the voltage of the source.

Basically, device 10 can be characterized as including a thermallyresponsive member, at least a portion of which tends to move as aconsequence of changes in the temperature thereof. Current modulatingmeans, comprising a pair of electrical contacts in the illustratedarrangement, are associated with this member so as to respond to thattendency to move. The current modulating means serve to increase theheat energy supplied to the thermally responsive member in response todecreases in temperature thereof, and vice versa. Consequently,throughout at least a predetermined range of voltages of the source, thecurrent modulating means periodically increase and decrease the energysupplied to the thermally responsive member and cause it to bemaintained at a substantially uniform average temperature for any givenresistance of senser 12. On this basis, it will be appreciated that thethermally responsive member receives energy, in pulsating form, at anaverage rate which does not vary with variations in the line voltage.Consequently, the output voltage of the device it is in the form of aseries or succession of energy pulsations having an eifective voltage (avoltage producing the same heating eflect in a resistive load as adirect voltage of that same value) which is substantially independent ofvariations in the source voltage. Further information as to the designand theory of operation of units of this type will be found in UnitedStates Patent No. 2,835,885, granted May 20, 1958, to L. Boddy.

Device comprises a polymetallic (representatively bimetallic)thermo-responsive unit including a pair of leg portions 30 and 32representatively illustrated as lying substantially in a common planeand extending in spaced parallelism with one another in that plane. Oneend of leg portion 32 is anchored or fixed, as is indicated by theearthing symbol 34 (which does not connote electrical grounding) and theother end of leg portion 32 is joined to one end of leg portion 3% bymeans of a crosspiece 36. An upstanding flange 38 is provided toincrease the stiifness of crosspiece 36 to the point where thatcrosspiece will not deflect to any significant degree in response to themechanical or thermal forces to which the unit is subjected.

The other end of leg portion 30 carries an electrical contact 40, thatcontact being electrically as well as mechanically integral with legportion 3t). Contact 4-0 is adapted to cooperate with a fixed oradjustably fixed contact 42, and the unit is constructed so that thosecontacts are in firm engagement with one another when the temperaturedifferential between leg portions 30 and 32 is zero or substantiallyzero. It will be appreciated that the provision of a U-shape member orthe nature shown will permit compensation for ambient temperaturevariations since the effect upon the position of contact 4i) of heatingof leg portion 30 is the opposite of that produced by heating of legportion 32.

Differential heating of leg portions 30 and 32 is accomplished byheating means in the form of a heater winding 44 representativelydisposed in heat transfer relationship with the leg portion 30', andmore particularly, coiled around that leg portion. One end of heaterwinding 4-4 is electrically connected to leg portion 39 as at 46, andthe other end of that winding is connected to conductor 48 which isconnected through the senser 12 to conductor 28. The fixed or adjustablyfixed contact 42 is connected to conductor 26. As a result, heaterwinding 44 is connected in series with senser 12, across the secondaryWinding of transformer 22 each time that contacts 4th and 42 are closedso that current flows through those contacts, through the heater winding44 and through senser 12. The resultant heating of leg portion 30 causesthe contact-carrying end of that portion to deflect in a direction toseparate contact 40 from contact 4-2. When the contacts 40 and 42 areseparated, the energizing circuit for heater winding 44 is interrupted,and leg portion 30 commences to cool, deflecting in a direction to bringcontact 46 into re-engagement with contact 42 to re-establish theinitial conditions. Leg portion 3%) continues to deflect in alternatedirections with a small-amplitude motion, tending to maintain theeffective wattage input (for iany given resistance of senser 12) to theheater winding 44 constant. As a result, the heater winding 44 willreceive energy as a series of pulsations the average or effectivevoltage of which does not vary with variations in the magnitude of thevoltage of source 10, and a pulsating voltage will appear between leg3t), and any parts electrically integral therewith, and conductor 28.Since the resistance of wire 44 is fixed, the effective current throughheater 44 and hence through senser 12 will also not vary in magnitudewith supply voltage variations. The effective voltage appearing betweenpolymetallic element 30 and conductor 28 and hence between outputconductor 50 and conductor 28 equals the sum of a fixed voltage dropacross heater 44 and a variable voltage drop across senser 12, theformer being determined by the product of the fixed effective currentand of the fixed resistance of heater 44 and the latter being determinedby the product of the fixed current and the variable resistance ofsenser 12. Therefore, the effective output voltage from device 10 doesnot vary with changes in the voltage of source 2t but does vary directlywith the resistance of senser 12. This output voltage appears betweenconductors 5t) and 28.

While it was assumed in the foregoing discussion that the resistance ofheater 44 is constant, it will be recognized that a wire having apositive temperature coefficient of resistance may be employed for thatheater to vary the operating conditions if desired.

An arrangement for modifying the effect of the changes of the value ofthe sensing resistor upon the output voltage will be describedhereinafter. The output voltage appearing between conductors 5t) and 28is applied across heater winding 52 of the responding device 14, one endof wire 50 being connected to one end of heater 52 and the other end ofheater winding 52 being electrically connected to leg portion 54 of aU-shape polymetallic element also including leg portion 56 disposed inspaced parallelism with leg portion 54 and crosspiece 58 which isprovided with a flange 69. This polymetallic element is or may besimilar to the polymetallic element of the pulsing device 19. t

The tip of leg portion 56 is anchored as is represented by the earthingsymbol 64 in Fig. 1. Conductor 28 is connected to leg portion 56 andhence the entire polymetallic element is at the potential of conductor23. Therefore, since one end of heater 52, on leg portion 54, iselectrically connected to leg portion 54, it is also connected toconductor 23 through leg portion 54, crosspiece 58 and leg portion 56.

In the illustrated arrangement, an armature 66 and an electrical contact68 are secured to the free end of leg portion 54, electrical contact 68being electrically integral with leg portion 54. Contact 63 is matablewith an electrical contact 70 which is attached to the free end of anangled spring 72, the other end of which is anchored or fixed.

The device 14 integrates the pulsations of energy which are applied toit from device it and responds only to the efiective value of thevoltage between conductors S and 28 and a spread or differential existsbetween the eflective voltage at which the contacts 63 and 7d arebrought into engagement and the higher voltage at which contacts 68 and7d are separated. In the illustrated embodiment, this spread isestablished by employing a permanent magnet to produce a snap action ofthe contacts, the permanent magnet in this case also serving as thecontact 79. This magnet cooperates with armature 66 so as to restrainseparation of the contacts 68 and 7t those contacts being engaged, as aresult of the pretension or bias in the polymetallic element of thedevice 14, when the leg portions 54 and 56 are of equal tempera tures.

In response to an increase in the effective voltage appearing acrosswinding 52, leg portion 54- heats, tending to deflect in a direction toseparate contact 68 from contact 78. This separation is inhibited by themagnetic action. However, when the efiective voltage rises to asuflicient value, the contacts 68 and 78 abruptly separate. Ifthereafter the effective voltage applied across windlng 52 reduces, thetemperature of leg portion 54 is lowered, and contact 68 approachescontact 70. When contact 68 is in a selected degree of proximity tocontact 78, the magnetic attraction between permanent magnet 78 andarmature 66 produces a rapid movement of the contact 68 into engagementwith contact 78.

The system acts thermostatically, an increase in the temperature sensedby senser 12 producing (with a posi tive temperature coeflicient ofresistance senser) an increase in the output voltage between conductors58 and 28 of a sufiicient magnitude to produce separation of contacts 68and '78. These contacts will remain separated until the sensedtemperature reduces sufliciently to drop the eifective voltage betweenconductors 58 and 28 to a value such that contact 68 will approachcontact '78 suificiently closely to produce snap closure. The contactswill remain closed until such time as the sensed temperature again risesto the selected higher value.

It will be appreciated that it is not imperative to the practice of theprinciples of the present invention that contacts 68 and 78 be snapacting or that the element 14 integrate the pulses but such anarrangement is presently preferred.

The sensed temperatures at which the contacts 68 and 78 are separated orclosed is adjusted, in the illustrated arrangement, by adjusting thepositon of contact 78. To this end, a screw 74 is threaded through anaperture at the elbow of the spring 72 and engages the face of a cam 76and hence serves as a cam follower to adjust the position of contact 78in accordance with the rotational position of cam 76. Cam 76 is manuallyrotatable in any suitable fashion.

Alternatively, the system may be adjusted electrically as by insertingresistors of selected values in conductor 58.

The closure of contacts 68 and 70 completes a circuit from conductor 28,through those contacts, conductor 78, sag wire 88 of the output relay 16which is representatively illustrated as a hot wire relay, and viaconductor 26 to the other end of the secondary winding of transformer22.

The hot wire relay 16 which is energized upon the closure of contacts 68and 78 of device 14 further includes a spring 82 which is pretensionedor biased so as to tend to bring contact 84 carried thereby intoengagement with a contact 86. However, when the wire 88 is at normaltemperatures, it exerts a force upon the spring 82 serving to deflectthe spring so as to separate those eletcrical contacts. When contacts 68and 78 of device 14 close, the previously traced energizing circuit iscompleted whereby current flows through wire 88. Wire 88 heats andelongates and permits spring 82 to move contact 84 into engagement withcontact 86.

The hot wire relay 16 follows the operation of device 14, closure ofcontacts 68 and 78 of device 14 producing rapid closure of contacts 84and 86 and opening of contacts 68 and 78 terminating heating of hot wire88 and causing contact 84 to be separated from contact 86. When contacts84 and 86 close, a circuit is completed from conductor 2811, switch 21,load 18, through the contacts, and to conductor 28c to energize the load18.

The arrangement as thus far described is similar to that disclosed inthe above-identified patent application of Harry 1. Baker and referencemay be had thereto for a disclosure of a suitable physical structurebased upon a constructed unit. In that unit, heater winding 44 wasselected to have a resistance of 14 ohms, and the resistance of senser12 varied from ohms at room temperature to about ohms at 450 F. The unitwas adjusted so that the relatively constant current through winding 44-and senser 12 was about milliamperes with a supply voltage (across thesecondary winding of the transformer) of 10 volts. Heater winding 52 wasselected to have a value of about 42 ohms. With these values, it wasfound that the output voltage of device 18 between conductors 58 and 28changed from about 4 volts to about 6 volts with changes in theresistance of senser 12 from 10 ohms to 20 ohms.

The present invention pertains to the modification of the magnitude ofthe change of output voltage of the device 18 with changes in theresistance of senser 12. In the illustrated arrangement, this isaccomplished by disposing a heater winding 98 in heat transferrelationship with leg portion 32 of the polymetallic element of device18 and connecting this winding so that it is responsive to the outputvoltage of device 18 as it appears between conductors 58 and 28. Asillustrated, heater Winding 98 is wound around leg portion 32 and oneend thereof is connected to leg portion 32 and hence to output condutcor58 and the other end of heater winding 98 is connected to conductor 28.

The output voltage between conductors 58 and 28 and hence the voltageacross winding 98 varies in accordance with the value of senser 12. Inthe illustrated arrangement, the magnitude of this voltage variesdirectly with sensed temperature.

Energization of winding 98 produces heating of leg portion 32, tendingto move the lower end thereof (the end remote from anchor 34) outwardlyfrom the plane of the paper. This movement produces a tilting of crosspiece 36 tending to rotate leg portion 38 so that its free end movesdownwardly from the plane of the paper. Consequently, heating of legportion 32 tends to move contact 48 toward contact 42, which is anopposite effect to that produced by energization of heater winding 44.

The thermal system including winding 98 and leg portion 32 is preferablyarranged to integrate the energy pulsations applied to the winding 98 asa result of the intermittent operation of contacts 48 and 42 so that thedegree of deflection of leg portion 32 in response to heating by winding98 varies in accordance with the average or eitective value of thevoltage appearing between conductors 58 and 28 as averaged over a numberof cycles of the pulsations of contacts 48 and 42.

While the energization of winding 98 produces a force tending to holdcontacts 4-8 and 82 together, the combination including contacts 48 and42, leg portion 38 and heater Winding 44 operates to maintain contacts48 and 42 in incipient condition of opening and closing. To re-establishthis condition, the average temperature of leg portion 38 must beincreased by an amount determined by the increase in temperature of legportion 32 resulting from the energization of winding 98. The increasein average temperature of leg portion 38 is accomplished by increasingthe wattage output of heater 44- as by increasing the duration of theclosed intervals of contacts 48 and 42. As a consequence, the eiiectivevalue of the current through heater winding 44 and hence through senser12 is increased and the effective voltage difference across the seriallyinterconnected elements 44- and 12 and hence between conductors 58 and28 is increased. Heater 98 senses and is subjected to this voltageincrease, its wattage dissipation being proportional to the square ofthe voltage applied thereacross. As a result, leg portion 32 tends to befurther heated, leg portion 38 must be provided with additional heat inorder for the contacts 48 and 42 to remain in their condition ofincipient opening and closing, the current through heater 4d and senser12 correspondingly increases, and the output voltage between conductors58 and 28 further increases. Thus, it will be seen that the thermalsystem including heater 98 and leg portion 32, connected as noted,serves as a positive or regenerative feedback arrangement, an increasein the effective voltage between conductors 58 and 28 as the result ofan increase in the resistance of senser 12 serving to produce a stillfurther increase in that output voltage, and vice versa.

The regenerative action does not continue to the point of loss ofcontrol or to the point of self-destruction of the unit becausethe powerin the heater winding 44 is always greater than the power in the heaterwinding 9%. As the resistance of senser 12 increases, the power inwinding 44- during each closed interval of contacts 40 and 42 decreases,tending to keep contacts 40 and 42 closed longer during eachclosed'interval, and the power in winding 96 will tend to furtherincrease the closed time of those contacts. However, since the power inwinding 44 is under all conditions greater than the power in winding 91the contacts 40 and 42 cannot fail to open and therefore regenerativedestruction cannot occur. If there is a change in ambient temperature,the heat losses due to radiation effects will not result in run-awayaction because the power in winding 44 will always remain the dominatinginfluence. This is because the fourth powers of the instant absoluteambient temperature and of the absolute reference (e.g., room)temperature of windings 44 and 90 are such that the power in winding 44is always greater than that in winding 94 regardless of the ambienttemperature.

Despite the equilibrizing effect as above discussed, the system isarranged so that winding 9t] produces a greater effect at high sensedtemperatures than at low sensed temperatures so as to produce aneffective amplification of the change of output voltage with a change insensed temperature. In the preferred arrangement, the Wattage output ofwinding 94 is small relative to the wattage output of winding 44 andaccordingly the temperature of polymetallic leg portion 32 is below thatof polymetallic leg portion 30. At low sensed temperatures, theresistance of senser 12 is low, the closed periods of contacts 40 and 42are relatively short, the output voltage between conductors 50 and 28 isrelatively low and the energy dissipated by heater 90 is relatively low,producing but a small elevation of the temperature of leg portion 32 andbut a relatively small increase in the output voltage between conductors50 and 28 at equilibrium conditions. On the other hand, at high sensedtemperatures, the resistance of senser 12 is relatively high, contacts4i) and 42 are closed for longer intervals of time, the output voltagebetween conductors 50 and 28 is relatively high, the heat energysupplied to leg portion 32 by heater 90 is relatively large, thetemperature of polymetallic leg portion 32 is elevated to a substantialdegree and contact 40 is thereby additionally biased toward contact 42by a sufiicient amount to produce a substantial increase in the outputvoltage appearing between conductors 50 and 28.

In a constructed embodiment of the invention, with winding 44 having avalue of about 14 ohms as above noted, winding 90 was selected to have aresistance of 200 ohms. With winding 90 omitted and with a voltpotential across the secondary winding of transformer 22, the outputvoltage change in response to a change of senser resistance from 10 toohms was 2 volts (from 4 volts to 6 volts). With winding 9%" operating,the output voltage changed a total of 6 volts (from 4 volts to 10 volts)in response to the same change in resistance of senser 12. In thatconstructed arrangement, the addition of heater 9% produced nodiscernible or appreciable affect upon the output voltage when thesenser was at its low resistance value but served to cause the outputvoltage between conductors 5th and 28 to rise to a point closelyapproaching the voltage across the secondary winding of transformer 22when senser 12 was at its higher value of resistance.

The amount of the positive feedback can be controlled by varying thesizes of the elements and particularly of the heat output relationshipsof windings 90 and 44. Excessive feedback can, of course, produceinstability and in the disclosed arrangement it will be observed thatthe lowering of the'resistance of winding 90 would tend to cause theoutput voltage between conductors and 28 to reach an eifectivemaximumvalue (determined 'by the secondary voltage of transformer 22) at alower temperature than the upper limit of the selected range ofoperation of the device.

It willalso be appreciated that since the output voltage is equal to thesum of a voltage drop across winding 44 which does not change to anysubstantial degree with changes in sensed temperature and of a voltagedrop across senser 12 which does change with sensed temperature, theeffect of unit change in senser resistance can be increased by reducingthe resistance of winding 44 relative to the resistance of senser 12.The design limitation upon increasing the sensitivity of the unit inthis fashion is normally found in the power dissipating capabilities ofsenser 12. In a constructed embodiment, however, it was found that theunit still operated satisfactorily with the resistance of heater winding44 reduced to a value as low as 2 ohms.

As above noted, in the embodiment of the invention illustrated in Fig.1, a spread or differential is established between the responder-windingvoltage at which contacts 68 and will close and the responder-windingvoltage at which those contacts will open. In the disclosed arrangement,this spread is achieved by the provision of a permanent magnet. Such aspread can alternatively be accomplished by providing a self-energizedlock-in winding on the relay 14 as by disposing an additional heaterwinding in heat transfer relationship with polymetallic leg portion 56of relay 14 or an additional heater winding in heat transfer relationwith leg portion 54 of device 14 and electrically connecting theadditional heater winding so that it is energized when contacts 68 and70 are closed if it is disposed upon leg portion 56 and de-energized (asby being shorted out) when contacts 68 and 70 are closed if theadditional heater winding is'placed upon leg portion 54. In the formercase, the heater winding on leg portion 56 can be electrically connectedin series with hot wire (as by being connected in conductor '78) or canbe connected in parallel with hot wire 80. In the latter case, theadditional heater winding, disposed in heat transfer relation withpolymetallic leg portion 54, can be electrically connected betweenconductors 78 and 28. The resistance of the additional winding should beselected so as to avoid an improper influence upon the current throughwire 80 and upon the action of hot wire relay 5%. It will be recognizedthat these modifications are applicable whether winding is or is notemployed to produce the above described amplifying effect and isapplicable whether the senser '12 is in series with the Winding'44 or isserially connected in conductor 59.

The modified arrangement illustrated in Fig. 2 of the drawings isbasically similar to the unit of Fig. 1 and corresponding elements havebeen designated with corresponding reference characters sufiixed by aprime symbol. This modified arrangement, however, includes an additionalheating coil 94 disposed in heat transfer relationship with polymetallicleg portion 32' of device 19. This coil is electrically connected incircuit with hot wire fit) so as to be energized and de-energizedconcurrently therewith as by being connected in series with that hotwire, as shown, or if desired, in parallel therewith. In accordance withthe teachings of the above-identified application of Tore B. Hanssen,winding 24 is adapted and serves partially or wholly to compensate forthe differential established by the permanent magnet 70 (or thecounterparts thereof as above discussed) or if desired, toovercompensate for that differential so as to cause the system includingdevices 10' and 14' to operate pulsatingly even in the absence ofchanges of sensed temperature. It will be observed that under thiscontemplated arrangement, the electro-thermalf'system including winding94 and'a portion of polymetallic le'g'po'r tion 32 is not intended tointegrate the pulsations of energy applied thereto as a result of theintermittent operation of contacts 68 and 70', whereas heater winding 90is intended, as above discussed, to integrate the higher frequencypulsations produced by the intermittent operation of contacts 40' and 42over a period of time which is large relative to the period of operationof contacts 4i)" and 42 (e.g., one minute).

it will .be perceived that the foregoing principles may also be appliedto the obtaining of efiective negative feedback so that the change inoutput voltage in response to unit change in the resistance of thesensing element would be reduced, again embodying the nonlinearcharacteristic whereby the effective reduction in the output voltage atany given value of resistance of the senser will vary to a substantialextent over the range of resist ances of the senser. Such negativefeedback can be advantageously employed, for example, to provideimproved linearity or to reduce the efi'ective sensitivity of thesensing element.

While the polymetallic elements of devices and 14 have been illustratedto be U-shaped and ambient compensated, the provision of such ambienttemperature compensation or the achievement of ambient temperaturecompensation in this particular fashion is not essential to the practiceof all of the principles of the present invention.

While it will be apparent that the embodiments of the invention hereindisclosed are well calculated to fulfill the objects of the invention,it will be appreciated that the invention is susceptible tomodification, variations and change without departing from the properscope or fair meaning of the subjoined claims.

What is claimed is:

1. In a system energizable from a source of voltage for actuating avoltage responsive device in accordance with a condition, thecombination of resistance means having a resistance which changes withchanges of the condition, a pair of electrical contacts, polymetallicmeans for controlling said contacts, means including said contacts forcontrolling the voltage responsive device, first electrical heatingmeans for said polymetallic means, means including said electricalcontacts and said resistance means for controlling the energization ofsaid first heating means from the source, second heating means for saidpolymetallic means, and circuit means including said electrical contactsfor controlling the energization of said second heating means from thesource.

2. The combination of claim 1 in which said first electrical heatingmeans and said electrical contacts produce self-interrupting operationfor causing said electrical contacts repetitively to open and close andin which the repetitive opening and closing of said contacts producespulsating energization of said second heating means.

3. In a system energizable from a source of voltage for actuating avoltage responsive device in accordance with a condition, thecombination of resistance means having a resistance which changes withchanges of the condition, a pair of electrical contacts, first andsecond polymetallic means for controlling said contacts and havingopposite effects upon the relative positions of said electricalcontacts, means including said contacts for controlling the voltageresponsive device, first electrical heating means for said firstpolymetallic means, means including said electrical contacts and saidresistance means for controlling the energization of said first heatingmeans from the source, second heating means for said second polymetallicmeans, and circuit means including said electrical contacts forcontrolling the energization of said second heating means from thesource.

4. The combination of claim 3 in which the voltage applied across saidsecond heating means varies with changes in the resistance ofsaidresistance means.

5. The combination of claim 3 in which said first and 10 secondpolymetallic means are the two legs of a generally U-shaped polymetallicelement, in which the tip of one of said legs is anchored and in whichthe tip of the other one of said legs moves one of the electricalcontacts.

6. The combination of claim 3 in which said first electrical heatingmeans and said electrical contacts produce self-interrupting operationfor causing said electrical contacts repetitively to open and close andin which the repetitive opening and closing of said contacts producespulsating energization of said second heating means and in which saidsecond polymetallic element and said second heating means effectivelyintegrate said pulsations over a period of time so that said secondpolymetallic element is heated and deflected to a degree determined bythe average value of the energy of the pulsations over a period of time.

7. The combination of claim 3 in which said electrical contacts areconnected in series with each of said heating means.

8. The combination of claim 7 in which said heating means are connectedin parallel with one another with respect to the source.

9. The combination of claim 8 in which said second heating means is alsoconnected in parallel with said resistance means with respect to thesource.

10. The combination of claim 9 in which said first heating means andsaid resistance means are connected in series with one another.

11. The combination of claim 3 in which the voltage applied across saidsecond heating means is controlled conjointly by said first heatingmeans and said resistance means.

12. The combination of claim 3 in which said voltage responsive deviceincludes and controls a second pair of electrical contacts, in which athird heating means is dis posed in heat transfer relation with one ofsaid polymetallic means, and in which said second pair of electricalcontacts control the energization of said third heating means.

13. The combination of claim 6 in which said voltage responsive deviceincludes and controls a second pair of electrical contacts, in which athird heating means is disposed in heat transfer relation with saidsecond polymetallic means, and in which said second pair of electricalcontacts control the energization of said third heating means.

14. In a system for association with a source of energy for controllinga heater in accordance with the sensed temperature of a body, thecombination of a pair of matable electrical contacts having opened andclosed states, means including said contacts for controlling the heater,actuating means including a winding efiective when a voltage is appliedacross said winding having a magnitude greater than a first selectedvalue for changing said contacts to one of said states and effectivewhen a voltage is applied across said winding having a magnitude lessthan a second selected value for changing said contacts to the other oneof said states, a temperature sensing device comprising a resistorhaving a high temperature coefficient of resistance disposed in heattransfer relation with the body, means including said temperaturesensing device connected to the source for applying a voltage acrosssaid winding which repetitively varies between first and second finitemagnitudes, at least one of which is controlled by said temperaturesensing device and varies in accordance with the sensed temperature, andmeans for increasing the magnitude of the change of the voltage appliedacross said winding which occurs in response to a given change of theresistance of said sensing device in one portion of the range ofvariation of the resistance of said sensing device relative to thechange of the voltage applied across said winding which occurs inresponse to the same change of the resistance of said I. 1 sensingdevice in another portion of the range of varition of the resistance ofsaid sensing device.

15. In a system for association with a source of energy for controllinga load device in accordance with a sensed condition, the combination ofa first pair of matable electrical contacts having open and closedstates, means including said contacts for controlling the load,actuating means including a first polymetallic element and heating meansin heat transfer relationship with said polymetallic element andefiective when a voltage is applied across said winding having amagnitude greater than a first selected value for changing said contactsto one of said states and eifective when a voltage is applied acrosssaid winding having a magnitude less than a second selected value forchanging said contacts to the other one of said states, resistance meanshaving a resistance which changes with changes of the condition, asecond pair of electrical contacts, polymetallic means for controllingsaid second pair of electrical contacs, means including said second pairof electrical contacts for controlling said heating means, firstelectrical heater means for said polymetallic means, means includingsaid second pair of electrical contacts and said resistance means forcontrolling the energization of said first electrical heater means fromthe source, second electrical heater means for said polymetallic means,and circuit means including said second pair of electrical contacts forcontrolling the energization of said second electrical heater means fromthe source.

16. In a system for association with a source of energy for controllinga load device in accordance with a sensed condition, the combination ofa first pair of matable electrical contacts having open and closedstates, means including said contacts for controlling the load,actuating means including a first polymetallic element and heating meansin heat transfer relationship with said polymetallic element andeffective when a voltage is applied across said winding having amagnitude greater than a first selected value for changing said contactsto one of said states and efiective when a voltage is applied acrosssaid winding having a magnitude less than a second selected value forchanging said contacts to the other one of said states, resistance meanshaving a resistance which changes with changes of the condition, asecond pair of electrical contacts, polymetallic means for controllingsaid second pair of electrical contacts, means including said secondpair of electrical contacts for controlling said heating means, firstelectrical heater means for said poly metallic means, means includingsaid second pair of electrical contacts and said resistance means forcontrolling the energization of said first electrical heater means fromthe source, second electrical heater means for said polymetallic means,and circuit means including said second pair of electrical contacts andsaid resistance means for controlling the energization of said secondelectrical heater means from the source and for applying across saidsecond electrical heater means a voltage having an average value whichvaries with variations of the resistance of said resistance means.

17. In a system energizable from a source of voltage for actuating avoltage responsive device in accordance with a condition, thecombination of resistance means having a resistance which changes withchanges of the condition, a pair of electrical contacts, first andsecond polymetallic means for controlling said contacts and havingopposite effects upon the relative positions of said electricalcontacts, means including said contacts for controlling the voltageresponsive device, first electrical heating means for said firstpolymetallic means, means including said electrical contacts and saidresistance means for controlling the energization of said first heatingmeans from the source for varying the output voltage applied to thevoltage responsive device in accordance with the resistance of theresistance means, and positive feedback means for amplifying the efiectof a change of resistance of the resistance means upon the outputvoltage comprising second heating means for said second polymetallicmeans, and connecting means including said electrical contacts forcontrolling the energization of said second heating means from thesource.

18. The combination of claim 17 in which said second heating means isenergized by said output voltage.

19. The combination of claim 18 in which the wattage output of saidsecond heating means is small relative to the wattage output of saidfirst means and in which the effect of said second heating means uponsaid output voltage is minimal at relatively low values of outputvoltage and is substantial at relatively high values of output voltage.

20. In a system for association With a source of energy for controllinga heater in accordance with the sensed temperature of a body, thecombination of a pair of matable electrical contacts having opened andclosed states, means including said contacts for controlling the heater,actuating means including a winding effective when a voltage is appliedacross said winding having a magnitude greater than a first selectedvalue for changing said contacts to one of said states and elfectivewhen a voltage is applied across said winding having a magnitude lessthan a second selected value for changing said contacts to the other oneof said states, a temperature sensing device comprising a resistorhaving a high temperature coefiicient of resistance disposed in heattransfer relation with the body, means including said temperaturesensing device connected to the source for applying a voltage acrosssaid winding which varies in accordance with variations of theresistance of said temperature sensing device, and means for increasingthe magnitude of the change of the voltage applied across said windingwhich occurs in response to a given change of the resistance of saidsensing device in one portion of the range of variation of theresistance of said sensing device relative to the change of the voltageapplied across said winding which occurs in response to the same changeof the resistance of said sensing device in another portion of the rangeof variation of the resistance of said sensing device.

21. In a system for association with a source of energy for controllinga heater in accordance with the sensed temperature of a body, thecombination of a pair of matable electrical contacts having opened andclosed states, means including said contacts for controlling the heater,actuating means including a Winding effective when a voltage is appliedacross said winding having a magnitude greater than a first selectedvalue for changing said contacts to one of said states and effectivewhen a voltage is applied across said winding having a magnitude lessthan a second selected value for changing said contacts to the other oneof said states, a temperature sensing device comprising a resistorhaving a high temperature coefficeint of resistance disposed in heattransfer relation with the body, means including said temperaturesensing device and electrothermal self-interrupting means including apolymetallic element and a pair of contacts controlled by saidpolymetallic element connected to the source for applying a voltageacross said winding which varies in accordance with variations of theresistance of said temperature sensing device, and means for increasingthe magnitude of the change of the voltage applied across said Windingwhich occurs in response to a given change of the resistance of saidsensing device in one portion of the range of variation of theresistance of said sensing device relation to the change of the voltageapplied across said winding which occurs in response to the same changeof the resistance of said sensing device in another portion of the rangeof variation of the resistance of said sensing device comprising heatingmeans for said polymetallic element energizable from the source ofenergy under the control of said electrical contents of saidelectrothermal self-interrupting means.

22. The combination of' claim 21 in which said'e1ectrothermalself-interrupting means supplies said voltage as a series of relativelyhigh frequency pulses and in which said heating means integrates saidpulses.

23. The combination of claim 21 in which the average value of thevoltage applied across said heating means varies in accordance with theresistance of said sensing device.

24. The combination of claim 14 in which said means for applying avoltage across said winding includes electrothermal self-interruptingmeans including a polymetallic element and a pair of contacts controlledby said polymetallic element and in which said means for increasing themagnitude of the change of the voltage comprises heating means for saidpolymetallic element energizable from the source of energy under thecontrol of 14 said electrical contacts of said electrothermalself-interrupting means.

25. The combination of claim 24 in which the average value of voltageapplied across said heating means varies in accordance with theresistance of said sensing device.

References Cited in the file of this patent UNITED STATES PATENTS2,205,637 Srnulski June 25, 1940 2,479,319 Crowley et a1 Aug. 16, 19492,833,889 Boddy May 6, 1958 2,846,531 Baker Aug. 5, 1958 2,846,556Whinery Aug. 5, 1958 2,894,105 Long et a1. July 7, 1959 2,910,569 BoddyOct. 27, 1959

